The present invention relates to light sources and, more particularly, to a light source based on photonic crystal structures.
Optics are pervasive in modem life. Optical applications are increasingly found in information technology and telecommunications; health care and the life sciences; optical sensing, lighting and energy; manufacturing; and national defense. Many of these applications require efficient light sources.
Lighting applications require light sources with a broad color spectrum that matches well to the response of the human eye. Both thermal sources (e.g., incandescent lamps) and luminescent sources (e.g., fluorescent lamps) are used to generate broad-band illumination for lighting. However, incandescent lamps are both inefficient and have a relatively short lifetime. The luminous efficacy (radiant flux in the spectral region of interest divided by the total input power) of a 60 W incandescent lamp using a tungsten filament is only about 15 lumens/Watt. By comparison, the theoretical maximum luminous efficacy for high-quality white lighting using a broad spectral source is around 200 lumens/Watt. The luminous efficacy of the incandescent lamp is low because much of the light (around 90%) is emitted by the tungsten filament in the non-visible infrared (wavelengths longer than 760 nm) portion of the optical spectrum. Fluorescent lamps have higher luminous efficacy, but tend to provide harsher lighting with poorer color quality.
About one-fifth of the total U.S. electricity consumption is currently devoted to lighting. Therefore, small increases in efficiency can represent large savings in energy and the cost of artificial lighting. Steady advancements have been made in traditional incandescent, fluorescent, and high-intensity gas discharge lamps, providing improvements in the efficiency and color quality for both existing and new kinds of light sources. For example, new phosphors make the generation of different colors possible, improving the color rendering of fluorescent and discharge radiators. Infrared coatings have provided improvements in the efficiency of incandescent lamps. However, these light sources still suffer from low energy efficiency, poor human visual response, high cost, ease of maintenance, or difficulty in distribution. Therefore, the improvement of existing light sources, the development new kinds of light sources, and high-efficiency distribution systems remain important goals in the lighting industry.
On the other hand, many radiation applications require light sources providing quasimonochromatic light having a high radiant flux in a narrow spectral band. These applications typically necessitate the development of different lamps for specific ranges of wavelengths. High-pressure gas discharge lamps, which emit copious line radiation, are frequently used for such purposes. Incandescent bulbs have also been covered with a filter to narrow the bandwidth and produce colored lamps, but at the expense of a further reduction in their emission efficiency.
A relatively new kind of quasimonochromatic light source is the semiconductor light-emitting diode (LED). LEDs emit radiation over a small spectral band. Therefore, the early developed red LEDs have been primarily used for specialized applications, such as automotive indicator lights, traffic signals, and information displays. Because of recent technical breakthroughs, there has been increased interest in the use of solid-state lighting for general illumination. In particular, recently developed blue and green LEDs can be used to produce xe2x80x9cwhitexe2x80x9d light when mixed with the existing high-brightness red and yellow LEDs. Color mixing of single-color LEDs or phosphor excitation with blue or UV LEDs offer the possibilities of improved color rendering and novel ways of using LED light sources. Solid-state lighting based on LEDs may compete with traditional incandescent and fluorescent light bulbs, if manufacturing costs can be reduced and brightness and efficiency can be increased.
However, for many optical applications, there remains a need for an efficient light source having an output that can be easily tuned over a broad spectral range. The present invention provides a low-cost, efficient narrow-band light source based on photonic crystal structures. The spectral properties of the light source can be easily tuned by modification of the photonic crystal structure and materials. These photonic crystal light sources may have applications in optical telecommunications, information displays, energy conversion, sensors, etc. As with LEDs, a broad-band light source for lighting applications can be realized by color mixing of single-color photonic crystal emitters or by phosphor excitation.
The present invention is directed to a light source, comprising a photonic crystal having at least two dielectric materials that provide a periodic variation in the dielectric constant of the photonic crystal such that the photonic crystal exhibits an enhanced photonic density-of-states over a band of frequencies, and wherein at least one of the dielectric materials has a complex dielectric constant to provide enhanced light emission at the band of frequencies when the photonic crystal is heated. The absolute value of the real part is preferably greater than about four times the imaginary part of the complex dielectric constant of the at least one of the dielectric materials at the band of frequencies. The absolute value of the real part of the complex dielectric constant is preferably greater than 10 and more preferably greater than 100. The imaginary part of the complex dielectric constant is preferably greater than 1 and more preferably greater than 5. The enhanced photonic density-of-states can occur within the allowed band, and preferably at the band edge of a photonic band gap of the photonic crystal. Alternatively, the enhanced photonic density-of-states can occur within the photonic band gap for a photonic crystal having a cavity or cavity-like defect. The light source can further comprise at least one additional photonic crystal having an enhanced photonic density-of-states at a different band of frequencies to provide for color mixing of the emitted light.
The present invention further includes a method for producing light emission, comprising heating the photonic crystal to provide enhanced light emission at the band of frequencies. The heating can comprise resistive heating by application of a bias voltage to the photonic crystal or other heating means. The photonic crystal can be thermally isolated from its surroundings to prevent thermal quenching of the optical emission.